A number of applications require a structural conductive carbon-carbon composite article or part such as for example aerospace brake pads. Such a carbon-carbon composite material is typically made by densifying carbon fiber mat or carbon bulk block by densification techniques such as for example by: (1) “infiltration” of a vapor phase into the carbon matrix such as a carbon fiber mat; or (2) “impregnation” of a liquid phase composition into the carbon matrix to form a preform followed by curing the preform and then eventually pyrolyzing the cured preform to produce a carbon-carbon composite article.
The objective of densifying a carbon-carbon composite part, for instance densifying the part from 10 lbs/cubic feet to 20 lbs/cubic feet, is to obtain a high carbon content in the part and to increase certain properties of the part. For example, the thermal conductivity of the part and/or the structural strength (e.g., maximum compressibility) of the part can be increased when the carbon content of the part can be increased. In applications such as aerospace brake pads, the thermal conductivity of the brake pads is critical and the thermal conductivity of the brake pads has to be sufficient to disperse efficiently the heat generated by friction exerted by the pads. Non-uniformities or defects in the carbon-carbon composite part used for the brake pads may decrease the reliability of the part.
Heretofore, manufacturers of carbon-carbon composites using a liquid phase precursor and the impregnation method described above have been faced with two choices. The manufacturer can employ a low-viscosity liquid matrix-precursor to obtain good impregnation under pressure of the low-viscosity liquid matrix-precursor into a carbon matrix and filling most large pores of the carbon matrix completely. However, this method results in a small amount of a low-quality matrix due to the low molecular weight of the liquid precursor and poor pyrolysis efficiency.
Alternatively, the manufacturer can utilize a high-viscosity liquid matrix-precursor forced into a carbon matrix with higher pressure which results in a small amount of a better-quality matrix due to the higher molecular weight of the liquid precursor and good pyrolysis efficiency. However, on the down side, the high-viscosity liquid matrix-precursor of this method does a poor job of filling the porosity and will degrade composite properties resulting from the increased breakage of the carbon material such as carbon fibers.
Generally, uniform densification requires a low viscosity (typically, lower than 100 mPa-s) and a high carbon yield (typically, above 40 percent (%)) with a carbon backbone structure typically provided by knitted or woven carbon fibers. Liquid epoxies can be the source of carbon rich material with more than 40% carbon content in the epoxies' molecular structure; but generally, such epoxies cannot be used for densification without a solvent because of the epoxies' high viscosity. In addition, the liquid epoxies do not yield solid structural carbon because of the epoxies' loose physical network created by the epoxies' molecular chains.
Japanese Patent Publication No. 29432/74 discloses a method for producing a carbon-carbon composite which includes the steps of: (I) mixing (A) organic fibers, such as pitch fibers, having a hydrogen/carbon atomic ratio of from 0.25 to 0.8, determined by elemental analysis, and etheric oxygen content of 3% to 15%, a carbonization yield of at least 50% but not more than 92%, preferably at least 70%; a linear shrinkage at 1,000° C. of 4% to 25%; a diameter of not more than 40 microns; and a fiber length/diameter ratio of at least 5; and (B) an organic binder, such as a phenolic resin or furfural resin, having a carbonization yield of at least 10%; (II) pre-shaping the mixture; and (III) firing the precursor. However, the method described in the above reference suffers from several disadvantages including for example (1) the method does not employ an epoxy resin formulation, (2) the formulation uses a solvent, (3) the viscosity of the resin is high, and (3) the carbon yield of the resulting fired precursor is low (e.g., not more than 10%).